System And Method For Cooling A Bog Stream

ABSTRACT

A system for cooling a boil-off gas (BOG) stream prior to compression in a boil-off reliquefaction plant, comprising a line ( 10 ) for feeding BOG into a compressor ( 11; 11 ″) prior to heat exchange with a closed-loop refrigeration system. The refrigeration system comprising compressors ( 2, 3, 4 ) and expanders( 8, 9 ) and a number of heat ex-changers for heat exchange with the BOG stream. The expanders ( 8, 9 ) are arranged in series. A precooler in the feed line ( 10 ) is fluidly connected ( 32, 33 ) to the closed-loop refrigeration system, whereby said BOG is pre-cooled in heat exchange with a portion of the coolant in the closed-loop refrigeration system prior to said compression.

The invention relates to the field of re-liquefaction of boil-off gases from liquid natural gas (LNG). More specifically, the invention relates to a method and system for cooling a boil-off gas stream, as set out in the introduction to the independent claim 1.

A common technique for transporting natural gas from its extraction site, is to liquefy the natural gas at or near this site, and transport the LNG to the market in specially designed storage tanks, often placed aboard a sea-going vessel.

The process of liquefying the natural gas involves compression and cooling of the gas to cryogenic temperatures (e.g. −160° C.). The LNG carrier may thus transport a significant amount of liquefied gas to its destination. At this destination, the LNG is offloaded to special tanks onshore, before it is either transported by road or rail on LNG carrying vehicles or re-vaporized and transported by e.g. pipelines.

LNG boils at slightly above −163° C. at atmospheric pressure, and is usually loaded, transported and offloaded at this temperature. This requires special materials, insulation and handling equipment in order to deal with the low temperature and the boil-off vapor. Due to heat leakage, the cargo (LNG) surface is constantly boiling, generating vaporized natural gas (“boil-off”)—primarily methane—from the LNG.

Plants for the continuous re-liquefaction of this boil-off gas are well known. The re-liquefaction of boil-off gases on LNG carriers results in increased cargo deliveries and allows the operator to choose the most optimal carrier propulsion system. LNG carriers have traditionally been driven by steam turbines, and the boil-off gases from the LNG cargo have been used as fuel. This has been considered a costly solution.

One such alternative to using the boil-off gas as fuel is the Moss RS™ Concept, wherein the boil-off gas is liquefied and the resulting LNG is pumped back to the cargo tanks. The Moss RS™ Concept, described in Norwegian Patent No. 305525 B1, is based on a closed nitrogen expansion cycle, extracting heat from the boil-off gas. Boil-off gas (BOG) is removed from the cargo tanks by two conventional LD compressors operating in series. The BOG is cooled and condensed to LNG in a cryogenic heat exchanger (“cold box”), to a temperature between the saturation temperature for compressed CH₄ and N₂ before being fed into a separator vessel where certain non-condensibles (mainly N₂) is removed. The LNG coming out of the separator is pumped back to the cargo tanks, while the non-condensibles (i.e. gases) are sent to a flare or vent stack.

The patented Moss RS™ concept has so far been designed for implementation onboard LNGC vessels ranging up to gross volumes of 216 000 m³. However, as newer and larger vessels are designed, the onboard power supply systems are not enlarged proportionally with the ship's physical dimensions. This provokes changes in process design in order to present more energy efficient solutions for the re-liquefaction of LNG boil-off gas.

The present Moss RS™ concept is based on a nitrogen Brayton cycle with three-stage compression and one-stage expansion. Using only one expander reduces the complexity of the compander-unit (compressors and expander) to a minimum, but the internal temperature approach between hot and cold streams is inadequately large in the middle sections of the cold-box. This is shown in FIG. 1. As the rather large area between the hot and cold composite curves represent a plain exergy loss, process innovations are sought in order to minimize this area.

The exergy losses can be reduced through the introduction of an additional expander. A recognized method for implementing such a unit is to split the refrigerant stream at a given temperature level and hence let two expanders work in parallel as described in Norwegian patent application 2004 0306.

It is, however, an object of the invention to reduce the exergy losses, without splitting streams. It is a need for a more efficient system that will improve performance and reduce the power demand.

The present invention meets that need, in that it provides a method for cooling a boil-off gas (BOG) stream prior to compression in a boil-off reliquefaction plant where the BOG stream following compression is reliquefied in heat exchange with a closed-loop refrigeration system comprising a coolant being compressed, before said reliquefied BOG being returned to a storage vessel, characterized by the following steps:

-   -   feeding compressed coolant into a first heat exchanger,     -   expanding the heat exchanged coolant,     -   feeding the expanded coolant into a second heat exchanger,     -   expanding the heat exchanged coolant,     -   feeding the expanded coolant into a third heat exchanger,     -   returning the coolant via heat exchangers to compressors.

The invention also provides a system for cooling a boil-off gas (BOG) stream prior to compression in a boil-off reliquefaction plant, comprising a line for feeding BOG into a compressor prior to heat exchange with a closed-loop refrigeration system, said refrigeration system comprising compressors and expanders and a number of heat exchangers for heat exchange with the BOG stream, characterized in that the expanders are arranged in series.

An embodiment of the invention will now be described in more detail, with reference to the accompanying drawings, where like parts have been given like reference numbers.

FIG. 1 shows a composite curve for the known one-expander Moss RS™ concept.

FIG. 2 is a principle flow diagram showing the Nitrogen Brayton cycle with two expanders in series, according to the invention.

FIG. 3 shows a composite curve for the nitrogen Brayton cycle with two expanders in series. (cf. FIG. 2).

FIG. 4 is a principle flow diagram illustrating the Nitrogen Brayton process with two expanders in series and nitrogen precooling according to the invention.

FIG. 5 shows a composite curve for the nitrogen Brayton cycle with the process according to the invention (cf. FIG. 4).

FIG. 6 is a principle flow diagram of an embodiment of the invention, illustrating pre- and intercooling LD compressor with parallel split streams.

FIG. 7 is a principle flow diagram of an embodiment of the invention, illustrating pre- and intercooling LD compressor with split stream in series.

FIG. 8 is a principle flow diagram of an embodiment of the invention, showing points for pre- and intercooling with intermediate-pressure nitrogen.

Implementing two expanders 8, 9 in series instead of in parallel, the exergy losses are reduced without splitting streams. The closed coolant (nitrogen) loop will then be similar to that of FIG. 2, where boil-off gas (BOG) from a reservoir (not shown) enters the compressor 11 through the line 10. The compressed stream 12 is routed through the heat exchanger 5, 6, 7 and heat exhanged against the closed-loop refrigeration system (this arrangement commonly referred to as a “cold-box”). With a careful tuning of the cold-box, the stream 13 exiting the heat exchanger(s) should be completely re-liquified.

Choosing two expanders in series, the exergy loss area is reduced by the introduction of a new local temperature pinch. This becomes clear when investigating the composite curve of FIG. 3, showing representative data for the two expanders 8, 9 in series cycle. Note that the overall duty is reduced with more than 1 MW compared to the process of FIG. 1, even though cooling capacity and temperature/pressure levels are maintained equal, seen from the BOG side.

The BOG is in most cases precooled before compression (by the compressor 11) prior to the cold-box. This is done in order to ensure a reasonable temperature profile in the cold-box and to achieve a more efficient compression in a reasonably sized LD-compressor-unit 11. The issue of precooling has been described in several other patents, involving methods such as

-   -   direct cooling with a fraction of the condensed BOG stream     -   indirect cooling with a fraction of the condensed BOG stream     -   indirect cooling with a fraction of the cold high pressure         nitrogen stream, taken out before entering the nitrogen expander         and throttled down to a lower pressure in a J-T expansion valve

To be able to liquefy most BOG compositions with a minimum of exergy losses, it is crucial to bring as much of the nitrogen refrigerant as possible down to the liquefier section of the cold-box. However, taking out nitrogen before (i.e. upstream of) the expander, will work against this principle. The same goes for the methods involving recycled liquefied BOG to be used in the precooling process. This will imply higher BOG flow rates at the low temperatures, and hence more circulated nitrogen will be necessary to cope with the increase in cooling demand at this temperature level.

In FIG. 4, the boil-off gas stream 10 from a reservoir (not shown) is compressed in a regular fashion in the compressor 11. The boil-off stream 12 is thus routed through a compact heat exchanger (visualized in the figures as four separate heat exchangers) 5, 6, 7, 7 b and heat exchanged against the closed-loop refrigeration system as will be described below. The stream 13 exiting the heat exchanger (or series of heat exchangers) is completely re-liquefied with a careful tuning of the refrigeration system. The person skilled in the art will appreciate that the heat exchangers 5, 6, 7, 7 b as shown in the figures, may be combined into one compact heat exchanger.

Turning now to the refrigeration system, FIG. 4 shows a compressor system comprising three in-line compressors 2, 3, 4. In a practical application, the compression may be achieved by one compression unit comprising three compressor wheels and two expander wheels connected via a common gear box. The compressor system compresses the coolant (refrigerant), e.g. nitrogen, and feeds this stream 15 into the first heat exchanger stage 5 where it is heat exchanged against the return coolant stream 20. After the first heat exchanger step 5, the coolant stream 16 is expanded in the expander 8 before being heat exchanged (stream 17) in the second heat exchanger 6 against the return coolant stream 20. The heat exchanged stream 17B is the heat exchanged in the third heat exchanger 7, before the stream 18 is expanded in the expander 9. The expanded coolant stream 19 is then heat exchanged in the fourth heat exchanger 7 b, then routed (line 20) back to the compressor system 2, 3, 4 through the heat exchangers 7, 6, 5.

As shown in FIG. 4, a BOG precooler 30 is included in the BOG feed line 10, upstream of the compressor 11. The line 33 feeds a fraction of the coolant (e.g. nitrogen) from a take-off point on the return coolant stream 20 between the second 6 and third 7 heat exchanger stage to the precooler 30, and the (heat exchanged) coolant is returned to the return coolant stream 20 via line 32, at an entry point downstream of said take-off point.

Thus, by using a fraction of the low-pressure nitrogen to precool the BOG stream, as in the invention, several favorable effects are seen:

-   -   1. The BOG flow rate through the cold-box is under normal         operation modes kept at the absolute minimum. When no recycling         of the LD-compressor is needed only direct boil-off from the         cargo tanks are processed in the reliquefaction system.     -   2. The refrigerant (or coolant, e.g. nitrogen) flow rate through         the low-temperature sections of the cold-box is the same as that         running through the 3 nitrogen compressors 2, 3, 4, i.e. the         maximum.     -   3. The fraction of the low-pressure nitrogen taken out from the         cold-box can be designed with an optimal temperature, ensuring         minimal exergy losses in the precooler 30.     -   4. Taking out some of the low-pressure refrigerant (e.g.         nitrogen) will result in a better match for the cold-box'         composite curves (i.e. reduced exergy losses) as this will         reduce the local temperature approach in an area where this         approach is generally large (ref. FIG. 5).

As a direct consequence of these measures the power demand of the invented process as illustrated in FIG. 4, can be reduced with additionally 100-150 kW. This reduction is reflected by the small step in the cold side composite curve, magnified in FIG. 5.

Another effect of splitting the low-pressure refrigerant stream is that it can be used, not only to precool the boil-off gas to the LD-compressor 30, but also to intercool the BOG between the two LD-compressor stages. This could potentially reduce the LD-compressor 11 work with around 50 kW (depending on amongst others the compressor efficiencies), but a slight increase in power demand to the nitrogen compander will equalize much of the power gained when considering the overall system. However, choosing such a solution offers more flexibility to adjust the temperature of the BOG entering the cold-box. This will, for different operational modes, reduce thermal stresses in the plate-fin heat exchanger, and open the possibility for reducing power under various operating conditions such as ballast voyages and voyages with nitrogen-rich LNG cargos.

Thus, a preferred embodiment and a flexible solution for integrating both precoolers and intercoolers is shown in FIG. 6. Here, two splits ensure that the BOG temperature can be cooled down to the same low temperature in both the precooler 30′ and the inter-cooler 30″. A BOG precooler 30′ is included in the BOG feed line 10, upstream of the first compressor 11′, while the BOG intercooler 30″ is included in the BOG feed line downstream of the first compressor 11″ and upstream of the second compressor 11′. The lines 33′, 33″ feed a part of the coolant (e.g. nitrogen) from a take-off point on the return coolant stream 20 between the second 6 and third 7 heat exchanger stage to the precooler 30′ and the intercooler 30″, respectively, and the (heat exchanged) coolant is returned to the return coolant stream 20 via lines 32′, 32″, respectively, at an entry point downstream of said take-off point.

It is also possible to choose only one split, as shown in FIG. 7. Here, the coolant is fed from the similar take-off point in the return line 20 as described above via line 37 to the precooler 30′, then via line 36 from the precooler 30′ to the intercooler 30″, before it is returned to the cold-box via line 38. This embodiment is only possible when higher temperatures are allowed in the intercooler compared to the aftercooler. It is, however, possible to reach intercooler temperature levels close to those of the precooler, but this implies a high nitrogen flow rate, and hence high pre- and intercooling exergy losses.

An alternative solution is to feed the cold nitrogen stream to the precooler from the intermediate-pressure nitrogen stream between the two expansion stages. This can in principle be done at any point between the two expanders, shown as points A, B, and C in FIG. 8. In point A, the local temperature approach is small, and the nitrogen flow rate might be slightly increased as a consequence of this, but points A and C will on the other hand make the plate-fin heat exchanger design somewhat less complicated than point B. The most suitable of the three points will be chosen as a result of economical considerations, control procedures, and energy demand of different LNG cargo compositions. For the already discussed two-expander solution, this will ensure that some of the expansion work is utilized for the nitrogen stream that is redirected to the precooler. 

1. A method for cooling a liquid natural gas (LNG) boil-off gas (BOG) stream (10) prior to compression by at least one compressor (11; 11′, 11″) in a boil-off reliquefaction plant where at least a portion of the BOG stream following compression is reliquefied in heat exchange with a coolant in a closed-loop refrigeration system comprising a heat exchanger unit (5, 6, 6B, 7) having at least two stages, the method comprising the steps of: feeding (33; 33′, 33″; 37) a fraction of said coolant from a take-off location (48) in the closed loop refrigeration system, to at least one heat exchanging location (30; 30′, 30″) upstream of the at least one compressor (11; 11′, 11″); bringing the coolant and the BOG stream into a heat exchanging relationship at the at least one heat exchanging location (30; 30′, 30″); and feeding (32; 32′, 32″; 38) the coolant from the at least one heat exchanging location (30; 30′, 30″), to a re-entry location (49) in the closed loop refrigeration system, downstream of said take-off location (48).
 2. The method of claim 1, wherein the BOG stream compression comprises a first BOG compression stage (11″), upstream of a second BOG compression stage (11′).
 3. The method of claim 2, wherein the coolant is brought into a heat exchanging relationship at a first heat exchanging location (30′) upstream of the first BOG compression stage (11″), and at a second heat exchanging location (30″) upstream of the second BOG compression stage (11′).
 4. The method of claim 2, further comprising the steps of: feeding (37) a fraction of said coolant from the take-off location (48) in the closed loop refrigeration system, to a first heat exchanging location (30′) upstream of the first BOG compression stage (11″); bringing the coolant and the BOG stream into a heat exchanging relationship at the first heat exchanging location (30′); feeding (36) the coolant from the first heat exchanging location (30′), to a second heat exchanging location (30″) upstream of the second BOG compression stage (11′); bringing the coolant and the BOG stream into a heat exchanging relationship at the second heat exchanging location (30″); and feeding (38) the coolant from the second heat exchanging location (30″), to the re-entry location (49) in the closed loop refrigeration system.
 5. The method of claim 1, wherein the fraction of the coolant is fed from a take-off location (48) between a third heat exchanger stage (6B) and a second heat exchanger stage (6) in the heat exchanger unit, and wherein coolant is fed to a re-entry location (49) between the third heat exchanger stage (6B) and the second heat exchanger stage (6) in the heat exchanger unit.
 6. The method of claim 5, wherein said take-off location (48) and said re-entry location (49) are located downstream of the third heat exchanger stage (6B) in the closed loop refrigeration system heat exchanger unit.
 7. The method of claim 1, wherein the fraction of the coolant is fed from a take-off location (48), and returned to the re-entry location (49), both locations in a first region (A) in the closed loop refrigeration system, downstream of a first heat exchanger stage (5) and upstream of a second heat exchanger stage (6) in the heat exchanger unit.
 8. The method of claim 1, wherein the fraction of the coolant is fed from a take-off location (48), and returned to the re-entry location (49), both locations in a second region (B) in the closed loop refrigeration system, downstream of a second heat exchanger stage (6) and upstream of a third heat exchanger stage (6B) in the heat exchanger unit.
 9. The method of claim 1, wherein the fraction of the coolant is fed from a take-off location (48), and returned to the re-entry location (49), both locations in a third region (C) in the closed loop refrigeration system, downstream of a third heat exchanger stage (6B) and upstream of a fourth heat exchanger stage (7) in the heat exchanger unit.
 10. An apparatus for cooling a liquid natural gas (LNG) boil-off gas (BOG) flowing in a BOG conduit (10), prior to compression by at least one compressor (11; 11′, 11″) in a boil-off reliquefaction plant where at least a portion of the BOG stream following compression is reliquefied in heat exchange with a coolant in a closed-loop refrigeration system comprising a heat exchanger unit (5, 6, 6B, 7) having at least two stages, said apparatus comprising: at least one heat exchanger (30; 30′, 30″) fluidly connected to the BOG conduit (10), upstream of the at least one compressor (11; 11′, 11″); a first line (33; 33′, 33″; 37) for feeding a fraction of said coolant from a take-off location (48) in the closed loop refrigeration system, to the at least one heat exchanger (30; 30′, 30″); and a second line (32; 32′, 32″; 38) for feeding coolant from the at least one heat exchanger (30; 30′, 30″), to a re-entry location (49) in the closed loop refrigeration system, downstream of said take-off location (48).
 11. The apparatus of claim 10, wherein the at least one compressor comprises a first BOG compressor (11″) arranged in the BOG conduit (10) upstream of a second BOG compressor (11′).
 12. The apparatus of claim 11, further comprising a first heat exchanger (30′) upstream of the first BOG compressor (11″), and at a second heat exchanger (30″) upstream of the second BOG compressor (11′).
 13. The apparatus of claim 12, wherein the first line (37) fluidly connects the coolant take-off location (48) in the closed loop refrigeration system with a coolant inlet of the first heat exchanger (30′); an intermediate line (36) fluidly connects a coolant outlet of the first heat exchanger (30′) with a coolant inlet of the second heat exchanger (30″); and the second line (38) fluidly connects a coolant outlet of the second heat exchanger (30″) with the re-entry location (49) in the closed loop refrigeration system.
 14. The apparatus of claim 10, wherein the take-off location (48) is located in a line (20′) between a third heat exchanger stage (6B) and a second heat exchanger stage (6) in the heat exchanger unit, and wherein the re-entry location (49) is located in the line (20′) between the third heat exchanger stage (6B) and the second heat exchanger stage (6) in the heat exchanger unit, downstream of the take-off location (48).
 15. The apparatus of claim 10, wherein the take-off location (48) and the re-entry location (49) are located in a first region (A) in a section (17) of the closed loop refrigeration system, downstream of a first heat exchanger stage (5) and upstream of a second heat exchanger stage (6) in the heat exchanger unit.
 16. The apparatus of claim 10, wherein the take-off location (48) and the re-entry location (49) are located in a second region (B) in a section (17B) of the closed loop refrigeration system, downstream of a second heat exchanger stage (6) and upstream of a third heat exchanger stage (6B) in the heat exchanger unit.
 17. The apparatus of claim 10, wherein the take-off location (48) and the re-entry location (49) are located in a third region (C) in a section (18) of the closed loop refrigeration system, downstream of a third heat exchanger stage (6B) and upstream of a fourth heat exchanger stage (7) in the heat exchanger unit.
 18. An apparatus for substantially reliquefying liquid natural gas (LNG) boil-off gas (BOG) in a boil-off reliquefaction plant, comprising a BOG feed line (10) and a compressor (11) fluidly connected to heat exchangers (5, 6, 7) and a return line (13) for re-liquefied BOG connected to ultimate outlet of the heat exchangers, wherein said heat exchangers the BOG is heat exchanged with a closed-loop refrigeration system comprising a coolant, said apparatus comprising: a compression unit (2, 3, 4) for compressing said coolant; a coolant line (15) fluidly connecting the compression unit with an inlet of a first coolant passage (51) of a first heat exchanger (5), said first heat exchanger also having a second coolant passage (52) and a BOG passage (53); a coolant line (16) fluidly connecting an outlet of the first heat exchanger's first coolant passage (51) with a first expander (8); a coolant line (17) fluidly connecting the outlet of the first expander (8) with an inlet of a first coolant passage (61) of a second heat exchanger (6), said second heat exchanger also having a second coolant passage (62) and a BOG passage (63); a coolant line (18) fluidly connecting an outlet of the second heat exchanger's first coolant passage (61) with a second expander (9); a coolant line (19) fluidly connecting the outlet of the second expander (9) with an inlet of a second coolant passage (72) of a fourth heat exchanger (7), said fourth heat exchanger also comprising a BOG passage (73); an outlet of said second fluid passage (72) of the fourth heat exchanger (7) being fluidly connected (20) to an inlet of said second fluid passage (62) of the second heat exchanger (6); an outlet of said second fluid passage (62) of the second heat exchanger (6) being fluidly connected to an inlet of said second fluid passage (52) of the first heat ex-changer (5); and an outlet of said second fluid passage (52) of the first heat exchanger (5) being fluidly connected to an inlet of said compression unit (2, 3, 4).
 19. An apparatus for substantially reliquefying liquid natural gas (LNG) boil-off gas (BOG) in a boil-off reliquefaction plant, comprising a BOG feed line (10) and a compressor (11) fluidly connected to heat exchangers (5, 6, 7) and a return line (13) for re-liquefied BOG connected to an ultimate outlet of the heat exchangers, wherein the BOG is heat exchanged with a closed-loop refrigeration system comprising a coolant, said apparatus comprising: a compression unit (2, 3, 4) for compressing said coolant; a coolant line (15) fluidly connecting the compression unit with an inlet of a first coolant passage (51) of a first heat exchanger (5), said first heat exchanger also having a second coolant passage (52) and a BOG passage (53); a coolant line (16) fluidly connecting an outlet of the first heat exchanger's first coolant passage (51) with a first expander (8); a coolant line (17) fluidly connecting the outlet of the first expander (8) with an inlet of a first coolant passage (61) of a second heat exchanger (6), said second heat exchanger also having a second coolant passage (62) and a BOG passage (63); a coolant line (17B) fluidly connecting an outlet of said first coolant passage (61) of the second heat exchanger (6) to an inlet of a first coolant passage (61B) of a third heat exchanger (6B); a coolant line (18) fluidly connecting an outlet of the third heat exchanger's first coolant passage (61B) with a second expander (9); a coolant line (19) fluidly connecting the outlet of the second expander (9) with an inlet of a second coolant passage (72) of a fourth heat exchanger (7), said fourth heat exchanger also comprising a BOG passage (73); an outlet of said second fluid passage (72) of the fourth heat exchanger (7) being fluidly connected (20) to an inlet of a second fluid passage (62B) of the third heat ex-changer (6B); an outlet of said second fluid passage (62B) of the third heat exchanger (6B) being fluidly connected (20′) to an inlet of the second coolant passage (62) of said second heat exchanger (6); an outlet of said second fluid passage (62) of the second heat exchanger (6) being fluidly connected to an inlet of said second fluid passage (52) of the first heat ex-changer (5); and an outlet of said second fluid passage (52) of the first heat exchanger (5) being fluidly connected to an inlet of said compression unit (2, 3, 4).
 20. The apparatus of claim 10, wherein the coolant comprises nitrogen.
 21. The apparatus of claim 10, for use on a floating vessel.
 22. The method of claim 1, wherein the coolant comprises nitrogen.
 23. The method of claim 1, for use on a floating vessel. 